The field of the present invention relates to bidirectional or multi-channel optoelectronic devices, including bidirectional optoelectronic transceivers. In particular, (i) a light source drive circuit, (ii) multi-function encapsulation, and (iii) a light-trapping structure formed on a waveguide substrate are disclosed herein for reducing cross-talk in a bidirectional optoelectronic device.
A bidirectional optoelectronic transceiver is a device that can simultaneously (i) receive one or more input optical signals and generate corresponding output electrical signals and (ii) receive one or more input electrical signals and generate corresponding output optical signals. More generally, a multi-channel optoelectronic device is one that can simultaneously handle such conversion between electrical and optical signals for two or more such pairs of corresponding signals (each pair comprising a “channel”). Such multi-channel devices can be “unidirectional” (i.e., wherein all input signals are optical and all corresponding output signals are electrical, or vice versa) or “bidirectional” (already described above).
In general, the input and output signals (optical and electrical) can be transmitted and received in any suitable way, including, e.g., free-space propagation (optical or electrical), electrical conduction by conductive wire, cable, or trace (electrical), or propagation as a guided mode in an optical fiber or waveguide (optical). It is common in telecommunications devices for the optical signals (input and output) to be received from or transmitted into an optical fiber or waveguide, and for the electrical signals to be received from or transmitted to a conductive wire, cable, or trace.
In this context, each signal (electrical or optical) typically comprises a carrier wave modulated according to a given scheme to encode digital or analog information (e.g., a digital data stream, an analog or digital video signal, or an analog or digital audio signal). The correspondence referred to above (i) between the input optical signal and the output electrical signal, and (ii) between the input electrical signal and the output optical signal, is a correspondence of the information encoded according to their respective modulation schemes. Many modulation schemes exist for encoding information onto an electrical or optical carrier signal. One common example of an electrical modulation scheme includes baseband digital amplitude modulation; another common example includes amplitude modulation of a radio frequency (RF) electrical carrier wave. One common example of an optical modulation scheme includes amplitude modulation of a visible or near-infrared optical carrier wave. Multiple electrical or optical modulation schemes can in some instances be used together or overlaid on one another. In some examples, by using differing carrier frequencies for input and output signals (electrical or optical), both input and output signals can be carried by a common transmission medium (e.g., input and output optical signals carried by a common optical fiber or waveguide, or input and output electrical signals carried by a common conductive wire, cable, or trace). In other examples, input and output electrical signals can be carried by separate conductive wires or traces, or input and output optical signals can be carried by separate optical fibers or waveguides.
Typically, care must be taken to limit the effects of cross-talk in a multi-channel or bidirectional optoelectronic device. Electrical cross-talk refers to an electrical signal (input or output) adversely affecting reception or generation of another electrical signal, and optical cross-talk refers to an optical signal (input or output) interfering with reception or generation of another optical signal. In principle, a cross-talk problem can arise in either or both directions (i.e., input affecting output, output affecting input, or both), and limiting cross-talk in both directions can be advantageous. In practice, in a bidirectional device, an input electrical signal (that drives the light source to generate the output optical signal) typically is larger in absolute magnitude than an output electrical signal (generated by photodetection of a typically weak input optical signal). As a result, the input electrical signal typically affects the output electrical signal (or its generation from the input optical signal) to a greater degree than the output electrical signal affects the input electrical signal (or generation of the output optical signal therefrom). Similarly, in a bidirectional device, the output optical signal typically is larger in absolute magnitude than the input optical signal. As a result, the output optical signal typically affects the input optical signal (or generation of the output electrical signal therefrom) to a greater degree than the input optical signal affects the output optical signal (or its generation from the input electrical signal).
Cross-talk in a multi-channel or bidirectional optoelectronic device can manifest itself in a variety of ways. In one example, electrical cross-talk can result in decreased sensitivity, in the presence of an input electrical signal, for reception of an input optical signal and generation of a corresponding output electrical signal by the photodetector. In another example, optical cross-talk can result in decreased sensitivity, in the presence of an output optical signal, for reception of an input optical signal and generation of a corresponding output electrical signal by the photodetector. In those examples and in others, such decreased sensitivity can manifest itself, e.g., as decreased signal-to-noise ratio, increased bit error rate for a digital signal, or increased noise floor. Sensitivity is simply the minimum optical power needed to ensure sufficiently faithful encoding on the output electrical signal of information encoded on the input optical signal (e.g., to guarantee a bit error rate below a specified limit for a digital data signal; various suitable criteria can be established for various types of signals). The sensitivity of the photodetector is typically degraded in the presence of an input electrical signal applied to the light source or the resulting output optical signal, relative to its sensitivity in the absence of an input electrical signal or output optical signal. Such degradation can be referred to or quantified generically as a “cross-talk penalty,” expressed as a ratio of the sensitivity of the photodetector with versus without the input electrical signal applied to the light source (or expressed as a difference between sensitivities given as dBm, for example). Reduction of optical or electrical cross-talk is a way to improve the photodetection performance of the bidirectional optoelectronic device, and can in some instances be imperative for meeting photodetection performance requirements of the device. Analogously, a cross-talk penalty can be quantified for faithful encoding on the output optical signal of information encoded on the input electrical signal in the presence of an input optical signal or an output electrical signal.